The subject matter disclosed herein relates to a rotary machine and, more particularly, to a tie bolt and stacked wheel assembly for the rotors of such machines.
The rotors of rotary machines, such as turbines and compressors, are typically formed of axially stacked wheels, which hold individual blades about their periphery. For example, compressor rotors include a series of individual compressor wheels stacked together with a set of tie bolts extending generally axially through the stack. The wheels mount the blades which, together with stator blades, form the compressor stages. The tie bolts are typically elongated studs threaded at both ends for receiving nuts to maintain the wheels in stacked, assembled relation relative to one another. Loosening of the nuts on the tie bolts reduces the tension on the bolts, thereby lowering the torque carrying capability of the rotor, eventually to unacceptable levels.
Traditionally, nuts are designed as hollow cylinders with internal threads. The circular face of the nut is pressed against a rotor surface to transfer a clamp load in a substantially axial direction. Positioning of the cylindrical nuts against the rotor surfaces requires additional space, thereby leading to an undesirable effect of the presence of a kink or step in the rotor midsection structure. The kink or step reduces the overall bending stiffness of the rotor and may lead to high gravity sag and high cycle fatigue stress.